1. Field of the Invention
The present invention relates to a telephone, and more particularly, to an apparatus for reducing echoes and noises.
2. Description of the Related Art
Generally in a wire or wireless system such as VoIP (Voice Over Internet Protocol) causing delay by a large amount, a transmitted signal evidently has echoes. As the echoes occur like this, when a speaker in a telephone outputs a voice of a far end talker, the voice is heard not only to a near end talker but also to the far end talker obstructing conversation since the voice is inputted via a microphone and then undergoes feedback toward the far end talker through several passages of a communication channel.
In a wire telephone in a Public Switched Telephone Network (PSTN), the created echoes rarely create any delay. On conversation via a handset, the echoes are not clearly heard or discriminated to a user covered with sidetone. Thus, the echoes may not cause any serious problems.
On the contrary, since a VoIP telephone or a mobile telephone may have the delay by a large degree, the quality of conversation is degraded due to the echoes in such a degree that it can be definitely recognized.
Classifying the echoes according to the cause of generation, the echoes include side-tone, induced echo, acoustic echo and the like, in which the side-tone is an echo generated by inputting a portion of a signal on a speech-transmitting again into a speech-receiving channel in the telephone for obtaining natural voice conversation, the induced echo is generated due to induction in a communication line or handset connector cord, and the acoustic echo is generated since a voice from a speaker in a receiving terminal is inputted into a microphone again.
In the above echoes, the sidetone is intentionally generated and thus has no problem. The acoustic echo is reduced by using an echo canceller with a Digital Signal Processor (DSP) in general.
In the telephone, the handset connector cord generally uses a flat-type curl cord having a pair of speech-transmitting lines and a pair of speech-receiving lines arranged in parallel. The cord is long enough to reach 2 or 3 m (meters) when the curl is unwound. Therefore, a signal in the speech-receiving line is induced to the speech-transmitting line by a large amount causing the echo so that the counterpart of the conversation can hear his or her own voice after a certain time period lapses.
The construction of a typical telephone connected to a handset is as follows. A telephone body and a handset are connected by a flat-type handset connector cord, in which only a speech-transmitting unit, a speech-receiving unit and a connecting unit of the telephone body are shown. The connecting unit has a pair of speech-transmitting line terminals Tx+ and Tx− and a pair of speech-receiving line terminals Rx+ and Rx−, in which the speech-transmitting line terminals Tx+ and Tx− are connected to the speech-transmitting unit. On the other hand, one of the speech-receiving line terminal Rx+ is connected to the speech-receiving unit, and the other one of the speech-receiving line terminal Rx− is connected to the ground. The speech-transmitting line terminal Tx+ is applied with a bias voltage Vref via a resistance R1. The handset connector cord joining the connecting unit to connect the handset to the telephone body includes a pair of speech-transmitting lines connecting to a microphone in the handset and a pair of speech-receiving lines connecting to a speaker in the handset. The handset connector cord uses a flat-type curl cord in which the Tx+ speech-transmitting line, the Rx+ speech-receiving line, Rx− speech-receiving line and the Tx− speech-transmitting line are arranged in sequence. Also, a modular jack is generally used to connect between the connecting unit and the handset connector cord.
In the handset connector cord as set forth above, the Rx+ speech-receiving line and the Tx+ speech-transmitting line are arranged side-by-side adjacent to each other so that a signal along the Rx+ speech-receiving line may be induced toward the Tx+ speech-transmitting line due to an induced current to generate the echoes. Further, the speech-receiving side is in the low impedance state and thus hardly influenced by external noises. On the other hand, the speech-transmitting side is in the high impedance state and thus readily influenced by even a small amount of the external noises.